An image printed by a printing press using toner is typically printed using a set of toners, comprising Cyan, Magenta, Yellow and optionally Black toners, commonly referred to as CMYK. Whereas black can theoretically be printed on a region of a printing substrate such as paper or transparent film by printing C, M and Y toners, blacks are often printed using a dedicated black toner, as a blacker black that requires less toner can generally be printed using a relatively inexpensive dedicated black toner rather than by printing a combination of C, M and Y toners.
Several types of toner are known, for example, powder toners, comprising from electrically charged pigmented particles in a powder form or liquid toners in which electrically charged pigmented toner particles are dispersed in a liquid carrier. In black toners carbon black is generally used to provide the black pigment of toner particles. However, blacks printed using toners comprising toner particles having only carbon black pigments are generally not completely neutral and may often evidence a degree of blue or brown tint. In addition, they have a tendency to fade and may develop a non-neutral hue with time. The eye is exceptionally sensitive to shades of gray and fading or drifts from a zero hue of printed blacks and grays as a result of exposure to light often affect quality of a printed image to a greater extent than fading of printed C, M or Y. In black liquid toners, in order to offset undesirable tint and moderate fading and drift from neutrality as a result of exposure; a colored pigment in addition to carbon black is often added to the toner.
UK Patent Application GB 2370 580 published on Jul. 3, 2002, the disclosure of which is incorporated herein by reference, describes black and gray inks for ink jet printing that comprise a carbon black; a phthalocyanine pigment (C.I. Pigment Blue 15.3 or 15.4); a dioxazine violet pigment (e.g. C.I. Pigment Violet 23); and an aqueous carrier medium. The application provides evidence that blacks and grays printed using the inks are freer from color tint than blacks and grays printed using prior art control inks. However, the inks are considerably less color neutral than black liquid toners known in the art. An example of one of the inks described in the application has CIELAB L*a*b* colorimetry values equal respectively to 21.7, 0.71 and 2.46 and an example of another of the inks has L*a*b* values equal to 52.3, 0.4 and 1.42. The application claims the inks show excellent light fastness but does not provide quantitative measures of the light fastness of the inks.